REACTIVATION HISTORY OF THE GAVILGARH-TAN MEGALINEAMENT AND DEVELOPMENT OF A MID-CONTINENTAL RIFT IN CENTRAL INDIA

A linear E-W to ENE-WSW trending mid-continental rift is formed by the Gavilgarh-Tan Megalineament (GTM) - a composite structure that occurs almost midway in the Central Indian Tectonic Zone (CITZ) that occupies middle part of the Indian Peninsular Shield. The term Gavilgarh-Tan Shear Zone is referred for the ductile component of the GTM that has affected Precambrian crystalline rocks and the Gavilgarh-Tan Faults (GTF) for all the subsequent dislocations under brittle regime. The ⁴⁰Ar/³⁹Ar dating of pseudotachylyte in the granitic basement in the Kanhan River section indicates two phases of pseudotachylyte generation at 831.6±6.7 Ma and 656.39±6.7 Ma. The Phanerozoic reactivation history of the GTF can be punctuated by incontrovertible evidence provided by deposition and faulting of Phanerozoic sequences. Geological record manifests nine distinct phases of reactivation since Neoproterozoic period.

The rift is developed along the Tapti-Purna section of the GTF that received periodic infilling of the Permo-Triassic Gondwanas, Late Cretaceous infra-trappeans and Deccan Traps, Tertiary and Quaternary sediments.

Evidence based on development of geological sequences, faulting and drilling indicate subsidence of graben floor by about 1000m in the pre-Quaternary period. The alluvial sediment-fill sequence in Purna valley attains a maximum thickness of 425m in the Quaternary period alone which is uniquely thick in the entire Peninsular India. Occurrence of salt-bearing sediments and highly saline water in the Purna valley bears testimony to the prevalence of intermittent hypersaline to evaporitic conditions in the Quaternary period. Drilling in Purna valley reveals that the Quaternary sediments have been deposited in a half graben structure.